The IP (Internet Protocol) Multimedia Subsystem (IMS) is a standardized architecture for providing multimedia services and voice-over-IP calls to both mobile and fixed user agents (UAs). The Session Initiation Protocol (SIP) been standardized and governed primarily by the Internet Engineering Task Force (IETF) as a signaling protocol for creating, modifying, and terminating IMS-based calls or sessions.
As used herein, the terms “user agent” and “UA” might in some cases refer to mobile devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices that have telecommunications capabilities. Such a UA might be part of a UE (User Equipment). A UE may have multiple UAs. A UE may have removable memory module associated, such as but not limited to a Universal Integrated Circuit Card (UICC) that includes a Subscriber Identity Module (SIM) application, a Universal Subscriber Identity Module (USIM) application, a IP Multimedia Services Identity Module (ISIM) application, or a Removable User Identity Module (R-UIM) application, etc. Examples of such modules could include, but are not limited to, PC Card, CompactFlash I, CompactFlash II, SmartMedia, Memory Stick, Memory Stick Duo, Memory Stick PRO Duo, Memory Stick PRO-HG Duo, Memory Stick Micro M2, Multimedia Card, Reduced Size Multimedia Card, MMCmicro Card, Secure Digital card, SxS, Universal Flash Storage, miniSD card, microSD card, xD-Picture Card, Intelligent Stick, Serial Flash Module, card, and NT Card. When the information is stored on a removable memory module, the contents of the module may be imaged into the UE.
Alternatively, such a UA might consist of the device itself without such a module. In other cases, the term “UA” might refer to devices that have similar capabilities but that are not transportable, such as fixed line telephones, desktop computers, set-top boxes, or network nodes. When one or more UAs are part of a network node, the network node could act on behalf of another function such as a UA or a fixed line device and simulate or emulate the UA or fixed line device. For example, for some UAs, the IMS SIP client that would typically reside on the device actually resides in the network and relays SIP message information to the device using optimized protocols. In other words, some functions that were traditionally carried out by a UA can be distributed in the form of a remote UA, where the remote UA represents the UA in the network. The term “UA” can also refer to any hardware or software component that can terminate a communication session that could include, but is not limited to, a SIP session. Also, the terms “user agent”, “UA”, “user equipment”, “UE”, and “node” might be used synonymously herein. Also, the terms “header” and “header field” might be used synonymously herein. Also, a SIP message is SIP request or a SIP response.
A UA might connect to a SIP-based network that includes a plurality of other components such as a P-CSCF (Proxy Call Session Control Function), an S-CSCF (Serving CSCF), an IBCF (Interconnect Border Control Function), an Application Server (AS), and other components, any of which could be referred to as network nodes. A trust relationship might exist between nodes in a SIP network. That is, a group of nodes within a network might regard all messages received from other nodes in the group as legitimate. Such a group can be said to form a trust domain or one or more trusted networks. IETF RFC 3325 titled “Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks” discusses this subject further.